Efficency of Converting Solar Irradiance into Electrical or Chemical Free Energy

1
Efficency of Converting Solar Irradiance into
Electrical or Chemical Free Energy

• A.J. Nozik
• National Renewable Energy Laboratory
• and
• Department of Chemistry, Univ. Colorado, Boulder

2
The U.S. Department of Energys National
Renewable Energy Laboratory
www.nrel.gov Golden, Colorado
3
FY02 EERE Funding at National Labs
Dollars in M
FY02 Budget Authority
4
Renewable Energy Cost Trends
Levelized cents/kWh in constant 20001
4030 20 10 0
100 80 60 40 20 0
PV
Wind
COE cents/kWh
1980 1990 2000 2010 2020
1980 1990 2000 2010 2020
70 60 50 40 30 20 100
1512 9 6 30
10 8 6 4 20
Solar thermal
Biomass
Geothermal
COE cents/kWh
1980 1990 2000 2010 2020
1980 1990 2000 2010 2020
1980 1990 2000 2010 2020
Source NREL Energy Analysis Office 1These graphs
are reflections of historical cost trends NOT
precise annual historical data. Updated October
2002
5
Solar Spectrum and Available Photocurrent
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? Solar Electricity ? Solar Fuels
7

8
National Geographic, Sept., 2004
9
World Energy
Millions of Barrels per Day (Oil Equivalent)
300 200 100 0
1860 1900 1940
1980 2020 2060
2100
Source John F. Bookout (President of Shell USA)
,Two Centuries of Fossil Fuel Energy
International Geological Congress, Washington
DC July 10,1985. Episodes, vol 12, 257-262
(1989).
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Photoeffects in Semiconductor-Redox Electrolyte
Junction Photoelectrochemistry (PEC)
C434703
Absorption of light in depletion layer results in
creation and separation of electron-hole pairs.
For n-type semiconductors, holes move toward
surface and electrons toward semiconductor bulk.
For p-type semiconductors, reverse process
occurs. Redox couples in electrolyte capture
injected photogenerated carriers and reactions
occur.
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SOLAR PHOTOCHEMISTRY/PHOTOELECTROCHEMISTRY
13
Some Endergonic Fuel Generation Reactions
14
SOLAR HYDROGEN--PHOTOELECTROLYSIS
15
Outstanding Technological Issues

• Discovery of Holy Grail of Photoelectrolysis
• Semiconductor with
• Bandgap ? 1/62.0 eV
• Appropriate flatband potential
• Catalytic surface for O2 (or H2) evolution
• Long-term stability against photocorrosion
• Conversion efficiency gt 10
• Low cost and environmentally benign

or
p-n combination of two different semiconductors
in a tandem configuration with above properties,
except bandgaps can be 1 eV.
16
Electrochemical Photovoltaic Cells
17
Dye-Sensitized Nanocrystalline TiO2 Photochemical
Solar Cell (Graetzel Cell)
Band Diagram
18
B084717
19
Main Process Limiting Conversion Efficiency
Hot e- Relaxation
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Detailed Balance Efficiency Calculation
The theoretical maximum efficiency of a solar
cell is calculated using the Detailed Balance
Model first introduced by Shockley and Queisser.
ASsUMPTIONS Absorption of one photon produces one
electron-hole pair. Quantum Yield 1. Only
photons with hn gt Eg are absorbed. Radiative
recombination is the only recombination
mechanism present. Hot carriers are relaxed to
the band edges The quasi-Fermi level separation
is constant through- out the cell. ? infinite
carrier mobility
Eg
EFn
V
EFp
J(V)
Load
Shockley and Queisser, J.Appl. Phys. 32, 510
(1961)
GBB blackbody photon flux
22
Net absorbed photon flux solar flux ambient
flux radiant emission flux
INET ABS (?) ?IS(?) IA (?)
I(?,µ,TQ,2p)s(?, µ,TQ) d? dA P INET ABS (?)
µ µ chemical potential produced by light ?Q
power converison efficiency ?Q INET ABS (?) µ
/ ? IS(?) h? d?
For single threshold absorber, maximum efficiency
?Q .31
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3rd Generation Photon Conversion Valid
Thermodynamic Approaches to Achieve Photon
Conversion Efficiencies gt 32 (Exceeding the
Shockley-Queisser Limit)

• 1. Tandem Cells (exceed S-Q limit but not new
  approach)
• 2. Hot Carrier Conversion
• Extract, collect, and utilize hot carriers
• Impact ionization/exciton multiplication
• Intermediate Band Solar Cell
• Thermophotonic Solar Cells
• Down conversion and upconversion of incident
  photons (M. Green and P. Wuerfel)
• See
• M. Green, Third Generation Photovoltaics.
  Springer, 2003
• A. Marti and A. Luque, Next Generaton
  Photovoltaics, Inst. Of Physics Series in Optics
  and Optoelectronics, 2003

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Efficiency of Hot Carrier Photoconversion
Ross Nozik, J. Appl. Phys. 53, 3813 (82)
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Multiple Threshold Absorbers
For an infinite number of tandem of tandem
absorbers
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2-PHOT0SYSTEM PEC CONVERSION
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Multi-Layered/Multi-Photon Photoelectrochemical
Converters (Photochemical Diode)
30
Wavelength Contours for Efficiency of Water
Splitting Utilizing Two Tandem Photosystems
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High Efficiency Multijunction Solar Cells

• Want 1eV material lattice-matched to GaAs
• ? Try GaInNAs

034016319
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Maximum Efficiency of Tandem Solar Cells
Calculated using a 6000K blackbody spectrum
34
Best Research-Cell Efficiencies
Spectrolab
36
Multijunction ConcentratorsThree-junction
(2-terminal, monolithic)Two-junction
(2-terminal, monolithic) Crystalline Si
CellsSingle crystalMulticrystalline Thin Film
TechnologiesCu(In,Ga)Se2CdTeAmorphous SiH
(stabilized) Emerging PVDye cells Organic
cells(various technologies)
Spectrolab
Japan Energy
32
NREL/ Spectrolab
NREL
NREL
28
UNSW
UNSW
24
UNSW
Spire
UNSW
NREL Cu(In,Ga)Se2 14x concentration
UNSW
Stanford
Spire
UNSW
ARCO
Georgia Tech
20
Efficiency ()
NREL
Sharp
Georgia Tech
Westing- house
Varian
NREL
NREL
NREL
16
NREL
UniversitySo. Florida
No. Carolina State University
NREL
Euro-CIS
Boeing
ARCO
Solarex
Boeing
12
Kodak
Boeing
United Solar
AMETEK
University ofLausanne
Masushita
United Solar
Kodak
Boeing
8
Monosolar
Photon Energy
RCA
Solarex
Boeing
Siemens
Groningen
University ofLausanne
University of Maine
Princeton
4
RCA
RCA
RCA
RCA
Cambridge
RCA
UCSB
UniversityLinz
RCA
Kodak
University Linz
0
Berkeley
2000
1995
1990
1985
1980
1975
2005
35
PV Module Production in 2003 by Technology Type
Source PV News, March 2004
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Photovoltaic Electrolysis
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Two-Junction Cascade PV/PEC Device for Water
Splitting
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Multi-Layered/Multi-Photon Photoelectrochemical
Converters (Photochemical Diode)
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John Turner Cell - gt 11 efficient water
splitting
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Projected Need for Carbon-Free Primary Power
Bottom Line New disruptive energy technology
is needed
41
From Martin Green
For PV or PEC to provide the level of C-free
energy required for electricity and fuelpower
cost needs to be 2-3 cents/kWh (0.40 0.60/W)
42
/peak watt (module cost/Eff ) (BOS cost/Eff)
0.1 where Eff cell conversion efficiency
x 1 Kw/m2 BOS balance of systems (support
structure, installation,wiring, land,
etc) 0.1 power conditioner, AC DC
inverter Also 1/Wp ? 0.05/kWh Therefore,
to achieve 0.02/kWh, need total cost of 0.40/
Wp If BOS can be reduced to 75/ m2
(currently ? 250/m2), and module cost reduced to
50/ m2 (currently ? 300/ m2 ), then module
efficiency needs to be 41 (and cell efficiency
at least 50). Disruptive technology required.
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Two Ways to Utilize Photogenerated Hot e- for
Useful Work and Increase Efficiency

• Higher photovoltage via hot e- transport,
  transfer, and conversion
• Higher photocurrent via carrier multiplication
  through impact ionization (inverse Auger process)

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Thermalized vs Hot Electron Transfer
Nozik, et. al. ,J. Applied Physics 54, 6463
(1983) Nozik Turner, Appl. Phys. Lett., 41, 101
(1982)
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Photocurrent Multiplication by Impact Ionization
h
1 photon yields 2 (or more) e- - h pairs (I.I.
previously observed in bulk Si, Ge, InSb)
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Maximum Single Bandgap Efficiency at 1 Sun
Impact Ionization
Impact Ionization
Detailed Balance
Shockley- Queisser limit
A. De Vos, B. Desoete, Solar Energy Materials
and Solar Cells 51 (1998) 413424
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Impact Ionization Processesin Bulk Semiconductors
Reverse biased p-i-n junction
Optically excited hot carriers
Electron initiated
Hole initiated
I
ETHgtEg
F
F
F
hn
I
Field
hngt2Eg
F
I
I
distance
I initial states
e- gain kinetic energy in a high electric field,
then scatter by II generating a secondary e-h
pair.
F final states
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Queisser, et al. 1994
Impact Ionization along the (100) direction (?
axis) of Si. Absorption of a photon h? creates a
first electron hole pair (e1/h1) at the ? point.
The excess energy Ex h? - Eg of the electron
suffices to generate a second electron hole pair
(e2/h2) while the electron e1 relaxes towards the
conduction-band minimum (e1). Conservation of
energy E and momentum hk/(2?) is fulfilled if the
two dash-dotted arrows add vectorially to
zero. QDs Requirement for conservation of
momentum is relaxed. Threshold should be lower.
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Consequences of Quantization

• Conversion of indirect semiconductors to direct
  semiconductors or vice versa
• Greatly enhanced exciton absorption at 300 K
• Greatly enhanced oscillator strength per unit
  volume (absorption coefficient)
• Greatly enhanced non-linear optical properties
• Greatly modified pressure dependence of phase
  changes and direct to indirect transitions
• Efficient anti-Stokes luminescence

• Dramatic variation of optical and electronic
  properties
• Large blue shift of absorption edge
• Discrete energy levels/structured absorption and
  photoluminescence spectra
• Enhanced photoredox properties for photogenerated
  electrons and holes
• Greatly slowed relaxation and cooling of
  photogenerated hot electrons and holes
• PL blinking in single QDs
• Enhanced impact ionization (inverse Auger
  recombination)

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Quantized Depletion Layers (w 50 to 200 Å)
(slower thermalization rates)
Hot e- injection APL (82) GaP JAP (82)
InP JACS (90) INP
Boudreaux, Williams and Nozik, JAP (1980)
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Hot e- Relaxation Pathways Quantum Films vs
Quantum Dots
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Breaking the Phonon Bottleneck in Quantum Dots by
an Auger-like Process involving a Coulomb
Interaction (Transfer of Electron Energy to Hole
Followed by Fast Hole Relaxation) (Efros)
Al. L. Efros et. al. Solid State Comm. 93, 281
(1995)
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Enhanced Photovoltaic Efficiency in Quantum Dot
Solar Cells by Inverse Auger Effect (Impact
Ionization)
Quantum Dot
A.J. Nozik, Physica E14,115, 2002 Ann. Rev.
Phys. Chem. 52, 193, 2001 in Next Generation
Photovoltaics, Marti Luque, Eds, AIP, 2003 in
Semiconductor Nanocrystals, V. Klimov, Ed.,
Marcel-Dekker, 2004
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Auger Ionization Process to Explain PL Blinking
in QDs
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Experimental Verification of Greatly Enhanced
Impact Ionization in Quantum Dots

• ? R.D. Schaller and V.I. Klimov, Phys. Rev.
  Letts, 92, 186601 (May), 2004 (PbSe QDs)
• ? R.J. Ellingson, M. Beard, P. Yu, A.J. Nozik,
  NanoLetters 5, 865, 2005 (PbSe and PbS QDs 300
  QY in PbSe QDs at 4 times Eg)

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Pump-probe transient absorption
Determine the photogenerated carrier density (QY)
and I.I. dynamics by (a) measuring the free
carrier absorption (IR probe) and exciton bleach
(HOMO-LUMO probe) (b) measuring dynamics of
multi-exciton decay vs single exciton decay, and
the rise time of exciton bleaching and induced
exciton absorption
Da a e-h pair (exciton) density 1S bleach decay
dynamics f(multiexciton density) 1S bleach
dynamics and induced exciton absorption determine
carrier cooling rate and carrier multiplication
rate
Pump hn gt nEg
IR Probe l5000nm HOMO-LUMO Probe ?
1300-1700 nm
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Transient Absorption Spectroscopy Setup
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QY gt 200 means 3 e-/photon are created QY
300 means alldots have 3 e- !!
NanoLetts 5, 865 (2005)
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NEW MODEL FOR MEG Coherent Superposition of
Multi-Excitonic States in PbSe QDs
NanoLetts 5, 865 (2005)
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SUMMARY/CONCLUSIONS

• ? The ultimate thermodynamic efficiency for
  converting solar irradiance into chemical or
  electrical free energy is 32 for a single
  thereshold absorber, and 68 for a system that
  does not permit thermal degradation of the solar
  photons. With full solar concentration (46,000X)
  the latter efficiency is 86.
• ? Ultra-high conversion efficiency (gt50)
  together with very low system cost (lt 150/m2) is
  required to produce solar power (fuels or
  electricity) at costs comparable to current
  fossil fuels cost (few cents/kWh), to avoid great
  economic and environmental disruption in the
  future. Disruptive technology is probably
  required.

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Summary/Conclusions

• ? Size quantization in semiconductors may
  greatly affect the relaxation dynamics of
  photoinduced carriers. These include
• - slowed hot electron relaxation (partial
  phonon bottleneck)
• - enhanced impact ionization (inverse Auger
  process)
• ? The theoretical and measured energy
  threshold for impact ionization in bulk
  semiconductors (e.g. Si, InAs, GaAs) is 4-5 times
  the band gap. Much lower thresholds are
  predicted for QDs because of the relaxation of
  the need to conserve momentum. The rate of
  impact ionization is also expected to be much
  faster in QDs (Auger processes a 1/d6 )
• ? Very efficient exciton multiplication has
  been experimentally observed in PbSe and PbS QDs
  the threshold photon energy is 2Eg. Up to 3
  electrons per photon (300 QY) have been
  observed at sufficiently high photon energies (?
  4Eg ). A new model based on coherent
  superposition of multiexcitonic states is
  introduced to explain these results.
• ? For QDs with me ltlt mh (InP) slowed
  electron cooling (by about 1 order of magnitude)
  may be achieved by either fast hole trapping at
  the surface or by electron injection in the dark,
  which blocks hot electron cooling via the Auger
  process(results consistent with earlier results
  on CdSe QDs by Guyot-Sionnest and Klimov). If
  me mh (PbSe and PbS) phonon bottleneck and
  slowed cooling is apparent.

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Summary/Conclusions - Continued
? Three configurations of Quantum Dot Solar
Cells are suggested 1. Nanocrystalline TiO2
sensitized with QDs 2. QD arrays exhibiting 3-D
miniband formation 3. QDs embedded in a
polymeric blend of electron- and hole-conducting
polymers. These configurations may be expected to
produce enhanced photovoltages via hot carrier
transport and transfer or enhanced photocurrents
via multiple exciton generation. ? THE DYNAMICS
OF HOT ELECTRON COOLING, FORWARD AND INVERSE
AUGER RECOMBINATION (MEG), AND ELECTRON TRANSFER
CAN BE MODIFIED IN QD SYSTEMS TO POTENTIALLY
ALLOW VERY EFFICIENT SOLAR PHOTON CONVERSION VIA
EFFICIENT MULTIPLE EXCITON GENERATION